This is probably because Strongbox’s session has become stale. You can clear this by going to
Preferences > Cloud > Sessions > Clear OneDrive session
Now you should be asked to login with OneDrive when you try to unlock your database. This will refresh the session and you should have no further issues.
It has been reported that sometimes even this doesn't help because the OneDrive app itself is installed on the device and it holds a stale session. In this case you should remove/re-add your accounts there and possible delete and reinstall the OneDrive app itself.